We hypothesize that the thermal characteristics of the bronchial circulation are different between asthmatic patients and normal individuals. When either group exercises, airway temperatures fall in a similar fashion. Cessation of exercise causes a rapid rewarming of the airway which is an order of magnitude faster in the asthmatic subjects. We speculate that this phenomenom is related to airway muscle or mucosal blood flow. Since tissue thermal conductivity is related to its blood flow, we propose to design and build a sensor using microelectronic fabrication techniques to evaluate the thermal characteristics of airway wall tissues. We propose to develop a surface probe consisting of temperature sensitive resistors, materials of known thermal conductivity and heated platinum thin film. By operating such a device at fixed temperatures, with the heated film as one element of a Wheatstone Bridge, we should be able to monitor rapid changes in tissue perfusion. This work will extend the collaboration which currently exists between my laboratory and the Resource for Biomedical Sensor Technology at the Electronics Design Center in the general area of intra-airway sensors. Our first pilot project, a sensor for determining airway surface fluid conductivity and temperature, is currently being used in animal studies to evaluate the impact of hyperventilation on surface fluid characteristics. This proposal, if funded, will allow us to determine the feasibility of fabricating and intra-airway sensor which will provide us with more information than temperature. There are no methods for the in vivo, dynamic determination of bronchial circulation blood flow in exercising human subjects.